There are several semiconductor photodetectors sensitive to wavelengths of 1.0 to 1.6 .mu.m for optical telecommunication systems, such as PIN photodetectors comprising a light absorbing layer of In.sub.0.53 Ga.sub.0.47 As lattice matched to an InP substrate disclosed on pages 653 to 654 of "Electron. Lett. vol. 20, 1984", or an avalanche multiplication semiconductor photodetector disclosed on pages 257 to 258 of "IEEE. Electron Device Lett. vol. 7, 1986". The avalanche photodector such as an avalanche photodiode has been used in long distance optical telecommunication systems, because it has an advantage in inner gain effects and high speed response due to avalanche multiplication.
One type of a conventional avalanche photodiode comprises an n-buffer layer formed on a substrate, an n-avalanche multiplication layer formed on the buffer layer, an n-light absorbing layer formed on the avalanche multiplication layer, and a p.sup.+ diffusion region formed on the avalanche multiplication layer.
In operation, a light is supplied to the avalanche photodiode which is applied with a reverse bias. The light is absorbed at the light absorbing layer to generate photocarriers, electrons and holes, of which the electrons are injected into the avalanche multiplication layer to cause ionization impacts which results in multiplication.
It is desirable that the ionization impacts which occur in the avalanche multiplication layer be carried out only by the photocarriers injected from the light absorbing layer. Therefore, it is desirable that the electron and hole ionization rates .alpha. and .beta. are vastly different ( .alpha. &gt; .beta. or .alpha. &lt; .beta. ) and that the photocarriers injected from the light absorbing layer initiate the avalanche process to provide an avalanche photodiode having low noise and high speed characteristics. The ratio .alpha./ .beta. depends on the property of material which the avalanche multiplication layer consists of. In an InGaAs type avalanche photodiode having an InP avalanche multiplication layer in which holes are injected carriers, for instance, the ratio .beta./ .alpha. of InP is up to 2 at the most, which is far smaller than the ratio .alpha./ .beta. of Si which is approximately 20.
Capasso et al have suggested that the ratio .alpha./ .beta. can be controlled artificially by using a superlattice structure having large band discontinuities as an avalanche multiplication layer, on pages 38 to 40 of "Appl. Phys. Lett. vol. 40, 1982".
In the wavelength band used for optical telecommunications (1.0 to 1.6 .mu.m), Brennan has analyzed theoretically based on Monte Carlo technique that the ratio .alpha./ .beta. of approximately 20 can be obtained by using InAlAs/InGaAs (In.sub.0.52 Al.sub.0.48 As/InGaAs in more exactitude) superlattice structure as an avalanche multiplication layer, on pages 1502 to 1510 of "IEEE. Trans. Electron Devices, ED-33, 1986". These avalanche photodiodes using the superlattice structure are expected to be superior to those using InGaAs system in the device performances.
According to the conventional avalanche photodiode using the superlattice structure, however, there is a disadvantage in that injected electrons may lose kinetic energy or may even be trapped at one end of a well layer on transferring the avalanche multiplication layer, so that the ionization rate .alpha.may decrease.
Brennan has suggested to provide an InAlGaAs graded region between well and barrier layers to reduce energy loss and trapping effects of electrons. However, it is very difficult to grow an InAlGaAs graded layer lattice matched with an InP substrate in light of crystal growth technique.